Strake systems and methods

ABSTRACT

There is disclosed a system comprising a structural element, at least one helical strake about the structural element, and at least one ramp to provide a transition from the structural element to the helical strake.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application60/713,981 filed on Sep. 2, 2005, herein incorporated by reference inits entirety.

FIELD OF INVENTION

The present disclosure relates to strake systems and methods.

BACKGROUND

Structural elements can be installed at sea from a floating vessel usinga J-lay configuration where the structural element is held vertically onthe vessel and dropped vertically into the water and then when itreaches the bottom of the body of water, it lays horizontal, oralternatively structural elements can be installed in a S-layconfiguration where the structural element is held horizontally on thevessel, drops to vertical through the body of water, and then rests onthe bottom of the body of water in a horizontal configuration. Otherconfigurations for installing a structural element from a vessel in abody of water are also known.

Referring now to FIG. 1, system 100 for installing structural element114 on bottom 116 of body of water 112 is illustrated. System 100includes vessel 110 with tensioner 120 and stinger 118. Tensioner 120holds structural element 114 in a horizontal configuration as it enterswater, and then structural element 114 rolls down stinger 118, thendrops to a vertical configuration, and then back to a horizontalconfiguration as it lays on bottom 116. Tensioner 120 and vessel 110have a sufficient capacity to support structural element 114 as it isbeing installed.

Currents in body of water 112 may cause vortexes to shed from the sidesof structural element 114. When these types of structural elements, suchas a cylinder, experience a current in a flowing fluid environment, itis possible for the structural element to experience vortex-inducedvibrations (VIV). These vibrations may be caused by oscillating dynamicforces on the surface which can cause substantial vibrations of thestructural element, especially if the forcing frequency is at or near astructural natural frequency. The vibrations may be larger in thetransverse (to flow) direction; however, in-line vibrations can alsocause stresses, which may sometimes be larger than those in thetransverse direction.

The magnitude of the stresses on a structural element is generally afunction of and increases with the velocity of the water current passingthese structural elements and the length of the structural element.

There are generally two kinds of current-induced stresses in flowingfluid environments. The first kind of stress is caused by vortex-inducedalternating forces that vibrate the structural element (“vortex-inducedvibrations”) in a direction perpendicular to the direction of thecurrent. When fluid flows past the structural element, vortices may bealternately shed from each side of the structural element. This producesa fluctuating force on the structural element transverse to the current.If the frequency of this harmonic load is near the resonant frequency ofthe structural element, large vibrations transverse to the current canoccur. These vibrations can, depending on the stiffness and the strengthof the structural element and any welds, lead to unacceptably shortfatigue lives. In fact, stresses caused by high current conditions inmarine environments have been known to cause structural elements such asrisers to break apart and fall to the ocean floor.

The second type of stress is caused by drag forces which push thestructural element in the direction of the current due to the structuralelement's resistance to fluid flow. The drag forces may be amplified byvortex induced vibrations of the structural element. For instance, ariser pipe that is vibrating due to vortex shedding will disrupt theflow of water around it more than a stationary riser. This may result inmore energy transfer from the current to the riser, and hence more drag.

Some devices used to reduce vibrations caused by vortex shedding fromsub-sea structural elements operate by modifying the boundary layer ofthe flow around the structural element to prevent the correlation ofvortex shedding along the length of the structural element. Examples ofsuch devices include sleeve-like devices such as helical strakeelements, shrouds, fairings and substantially cylindrical sleeves.Currently available strake elements and fairings cover an entirecircumference of a cylindrical element or may be clamshell shaped to beinstalled about the circumference.

Some VIV and drag reduction devices can be installed on risers andsimilar structural elements before those structural elements may bedeployed underwater. Alternatively, VIV and drag reduction devices canbe installed on structural elements after those structural elements maybe deployed underwater.

When installing a structural element in an S-lay configuration, thestructural element may travel over a stinger and encounter one or morerollers on the stinger. A pre-installed strake may be damaged if itpasses over the stinger. One alternative is to install the strakes onthe structural element after it passes over the rollers and the stinger.Another alternative is to protect the strakes as they are passed overthe rollers and the stinger.

U.S. Pat. No. 6,896,447 discloses a vortex induced vibration suppressorand method. The apparatus includes a body that is a flexible member of apolymeric (e.g., polyurethane) construction. A plurality of helicalvanes on the body extend longitudinally along and helically about thebody. Each vane has one or more openings extending transversely therethrough. A longitudinal slot enables the body to be spread apart forplacing the body upon a riser, pipe or pipeline. Tensile members thatencircle the body and pass through the vane openings enable the body tobe secured to the pipe, pipeline or riser. U.S. Pat. No. 6,896,447 isherein incorporated by reference in its entirety.

There is a need in the art for an improved apparatus and method forsuppressing vibration. There is another need in the art of apparatus forand new and improved methods of installing strake elements forsuppressing vibration in a flowing fluid environment. There is anotherneed in the art of apparatus for and new and improved methods ofinstalling strake elements for suppressing vibration in a flowing fluidenvironment on a structural element before the structural element isinstalled over a ramp or roller. There is another need in the art ofapparatus for and new and improved methods of installing strake elementsfor suppressing vibration in a flowing fluid environment on a structuralelement before the structural element is installed in the flowing fluidenvironment which does not require intervention or adjustment of thestrake elements once the structural element is in the flowing fluidenvironment.

These and other needs of the present disclosure will become apparent tothose of skill in the art upon review of this specification, includingits drawings and claims.

SUMMARY OF THE INVENTION

One aspect of the invention provides a system comprising a structuralelement, at least one helical strake about the structural element, andat least one ramp to provide a transition from the structural element tothe helical strake.

Another aspect of the invention provides a method of installing astructural element in a body of water comprising attaching at least onehelical strake about the structural element, attaching at least one rampto the structural element and/or the at least one helical strake, the atleast one ramp to provide a transition from the structural element tothe helical strake, and moving the structural element, the ramp, and thestrake over a roller, so that the at least one ramp provides atransition from the structural element to the helical strake where theroller interfaces with the structural element, the ramp, and the strake.

Advantages of the invention include one or more of the following:

-   -   improved apparatuses and methods for suppressing vibration;    -   improved methods of installing strake elements for suppressing        vibration in a flowing fluid environment;    -   improved methods of installing strake elements for suppressing        vibration in a flowing fluid environment on a structural element        before the structural element is installed over a ramp or        roller; and    -   improved methods of installing strake elements for suppressing        vibration in a flowing fluid environment on a structural element        before the structural element is installed in the flowing fluid        environment which does not require intervention or adjustment of        the strake elements once the structural element is in the        flowing fluid environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for installing a structural element in abody of water in an S-lay configuration.

FIG. 2 illustrates a system for installing a structural element in abody of water in an S-lay configuration.

FIGS. 3 a and 3 b illustrate a structural element with strakes.

FIGS. 4 a-4 c illustrate a structural element with strakes and rampstraveling over a stinger.

FIGS. 4 d and 4 e illustrate a structural element with strakes andramps.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, there is disclosed a system comprising a structuralelement, at least one helical strake about the structural element, andat least one ramp to provide a transition from the structural element tothe helical strake. In some embodiments, the structural element isselected from the group consisting of a shell, a collar, an oilflowline, a pipeline, a drilling riser, a production riser, a steeltubular, import and export risers, subsea pipelines, tendons for tensionleg platforms, legs for traditional fixed and for compliant platforms,space-frame members for platforms, cables, umbilicals, mooring elementsfor deepwater platforms, hull structures for tension leg platforms andfor spar type structures, and column structures for tension legplatforms and for spar type structures. In some embodiments, thestructural element comprises a plurality of sections welded to eachother. In some embodiments, the structural element comprises a pluralityof sections threaded to each other. In some embodiments, the at leastone helical strake about the structural element comprises at least threehelical strakes about the structural element. In some embodiments, theat least one ramp comprises a plurality of ramps aligned along alongitudinal axis of the structural element, the ramps adapted tointerface with a stinger and/or a roller. In some embodiments, the atleast one ramp comprises a first set of ramps and a second set of ramps,the first set and the second set aligned along a longitudinal axis ofthe structural element, the first set adapted to interface with a firstroller, and the second set adapted to interface with a second rollerazimuthally spaced apart from the first roller. In some embodiments, afirst end of the at least one helical strake is attached to a firstcollar, and a second end of the at least one helical strake is attachedto a second collar, the first collar and the second collar attachedabout the structural element.

In one embodiment, there is disclosed a method of installing astructural element in a body of water comprising attaching at least onehelical strake about the structural element, attaching at least one rampto the structural element and/or the at least one helical strake, the atleast one ramp to provide a transition from the structural element tothe helical strake, and moving the structural element, the ramp, and thestrake over a roller, so that the at least one ramp provides atransition from the structural element to the helical strake where theroller interfaces with the structural element, the ramp, and the strake.In some embodiments, the structural element is selected from the groupconsisting of a shell, a collar, an oil flowline, a pipeline, a drillingriser, a production riser, a steel tubular, import and export risers,subsea pipelines, tendons for tension leg platforms, legs fortraditional fixed and for compliant platforms, space-frame members forplatforms, cables, umbilicals, mooring elements for deepwater platforms,hull structures for tension leg platforms and for spar type structures,and column structures for tension leg platforms and for spar typestructures. In some embodiments, the structural element comprises aplurality of sections welded to each other. In some embodiments, thestructural element comprises a plurality of sections threaded to eachother. In some embodiments, attaching at least one helical strake aboutthe structural element comprises attaching at least three helicalstrakes about the structural element. In some embodiments, the at leastone ramp comprises a plurality of ramps aligned along a longitudinalaxis of the structural element, where the roller interfaces with thestructural element. In some embodiments, the at least one ramp comprisesa first set of ramps and a second set of ramps, the first set and thesecond set aligned along a longitudinal axis of the structural element,the first set adapted to interface with a first roller, and the secondset adapted to interface with a second roller azimuthally spaced apartfrom the first roller. In some embodiments, the first roller isazimuthally spaced apart from the second roller by 90 to 150 degreesmeasured as an arc angle of the structural element.

Referring now to FIG. 2, in one embodiment of the invention, system 200is illustrated. System 200 includes vessel 210 in body of water 212,installing structural element 204 in body of water 212 and resting aportion of structural element 204 on bottom 216. Vessel 210 may includetensioner 220 to keep tension on structural element 204 so that it doesnot sink in water 212. Strakes 206 are attached to structural element204 to dampen any vortex induced vibration of structural element 204.

Referring now to FIGS. 3 a-3 b, in some embodiments of the invention,structural element 304 is illustrated. Structural element 304 enclosespassage 302. Strake elements 306 a, 306 b, and 306 c may be mountedabout the circumference of structural element 304. Strake elements 306a-306 c serve to inhibit vortex induced vibration when structuralelement 304 is in a flowing fluid stream.

Structural element 304 has outside diameter D 328. Strake elements 306a-306 c have height H 330. Adjacent strake elements may be spaced apartby a pitch L 332. In some embodiments of the invention, outside diameterD 328 may be from about 2 to 60 cm. In some embodiments of theinvention, height H 330 may be from about 5% to about 50% of outsidediameter D 328. In some embodiments of the invention, height H 330 maybe from about 1 to about 15 cm. In some embodiments of the invention,pitch L 332 may be from about 1 D to about 10 D. In some embodiments ofthe invention, pitch L 332 may be from about 10 to about 500 cm.

In some embodiments of the invention, there may be about 1 to about 10helical strake starts about a circumference of structural element 304.In some embodiments of the invention, there may be about 2 to about 6helical strake starts about a circumference of structural element 304.In some embodiments of the invention, there may be about 3 helicalstrake starts about a circumference of structural element 304.

In some embodiments of the invention, strakes 306 a-306 c may be made ofa polymer, such as a thermoplastic polymer or a thermosetting polymer,for example polypropylene, polyethylene, other polyolefins, orco-polymers of olefins. In some embodiments of the invention, strakes306 a-306 c may be made of a composite, such as fiberglass or carbonfiber composite. In some embodiments of the invention, strakes 306 a-306c may be made of a metal, such as steel or aluminum.

In some embodiments of the invention, strakes 306 a-306 c may beattached to a collar, pipe, shell, or other support apparatus. Thesupport apparatus and strakes 306 a-306 c may then be installed aboutstructural element 304.

Referring now to FIGS. 4 a-4 c, in some embodiments of the invention,stinger 418 and structural element 404 are illustrated. Stinger 418includes roller 419 a and roller 419 b which are adapted to transportstructural element 404. Structural element 404 is able to roll downstinger 418 while resting on rollers 419 a and 419 b. In someembodiments of the invention, rollers 419 a and 419 b may be azimuthallyspaced from about 90 to about 150 degrees apart, measured as anarc-angle of structural element 404.

Referring now to FIG. 4 b, structural element 404 is shown incross-section moving along stinger 418. Structural element 404 enclosespassage 402 and has attached to its exterior strakes 406 a, 406 b, and406 c. Stinger has rollers 419 a and 419 b, which interface with anexterior of structural element 404 to support structural element 404 andallow structural element to roll along stinger 418.

Referring now to FIG. 4 c, in some embodiments of the invention,structural element 404 of FIG. 4 b has moved further along so thatstrake 406 b is interfacing with roller 419 b, and strake 406 c isinterfacing with roller 419 a. Ramps 408 b are provided adjacent strake406 b, and ramps 408 c are provided adjacent strake 406 c. Ramps 408 band 408 c are adapted to interface with rollers 419 a and 419 b to liftstructural element 404 and provide a smooth transition from the outsidesurface of structural element 404 to the height of strakes 406 b and 406c, so that the strakes are not damaged when they encounter the rollers.

Referring now to FIG. 4 d, in some embodiments of the invention, a sideview of structural element 404 is illustrated. Line 405 indicates whereroller 419 b encounters structural element 404. Ramps 408 a, 408 b, 408c, and 408 d are provided along line 405, to provide a smooth transitionof lifting and lowering structural element when it encounters roller 419b, so that strakes 406 a, 406 b, and 406 c are not damaged. Similarramps may be provided on the opposite side of structural element 404where roller 419 a encounters structural element 404.

Referring now to FIG. 4 e, in some embodiments of the invention, adifferent side view of the structural element 404 illustrated in FIG. 4d is shown. In this view, line 405 where roller 419 b encounters thestructural element is at the top, so that the tapering of ramps 408 a,408 b, 408 c, and 408 d may be seen. The ramps provide a smoothtransition from the outside surface of structural element 404 to theheight of each of the strakes, and then back to the outside surface ofthe structural element 404 to the roller 419 b, so that the strakes arenot damaged when they encounter the roller.

In some embodiments of the invention, strakes 406 a-406 c may beattached to a collar, pipe, shell, or other support apparatus. Thesupport apparatus and strakes 406 a-406 c may then be installed aboutstructural element 404. The ramps provide a smooth transition from theoutside surface of the support apparatus to the height of each of thestrakes, and then back to the outside surface of the support apparatusto the roller 419 b, so that the strakes are not damaged when theyencounter the roller.

In some embodiments of the invention, clamshell type strake elements maybe mounted around a structural element according to the method disclosedin U.S. Pat. No. 6,695,539, which is herein incorporated by reference inits entirety.

In some embodiments of the invention, strake elements may be installedabout a structural element according to the method disclosed in U.S.Pat. No. 6,561,734, which is herein incorporated by reference in itsentirety.

In some embodiments of the invention, strake elements may be installedabout a structural element according to the method disclosed in UnitedStates Patent Application Publication No. 2003/0213113, which is hereinincorporated by reference in its entirety.

In some embodiments of the invention, the outside diameter of astructural element to which strake elements can be attached may be fromabout 10 to about 50 cm. In some embodiments of the invention, theheight of strake elements may be from about 5% to about 50% of thestructural element's outside diameter. In some embodiments of theinvention, the height of strake elements may be from about 5 to about 20cm.

In some embodiments of the invention, the structural element may becylindrical, or have an elliptical, oval, or polygonal cross-section,for example a square, pentagon, hexagon, or octagon.

In some embodiments, portions of structural element 204 may be loweredonto bottom 216 of water 212. In some embodiments, water 212 has a depthof at least about 1000 meters, at least about 2000 meters, at leastabout 3000 meters, or at least about 4000 meters. In some embodiments,water 212 has a depth up to about 10,000 meters.

In some embodiments of the invention, structural element 204 may be apipeline, a crude oil flowline, a mooring line, a riser, a tubular, orany other structural element installed in a body of water. In someembodiments, structural element 204 may have a diameter from about 0.1to about 5 meters, and a length from about 10 to about 200 kilometers(km). In some embodiments, structural element 204 may have a length todiameter ratio from about 100 to about 100,000. In some embodiments,structural element 204 may be composed from about 50 to about 30,000tubular sections, each with a diameter from about 10 cm to about 60 cmand a length from about 5 m to about 50 m, and a wall thickness fromabout 0.5 cm to about 5 cm.

Those of skill in the art will appreciate that many modifications andvariations are possible in terms of the disclosed embodiments,configurations, materials and methods without departing from theirspirit and scope. Accordingly, the scope of the claims appendedhereafter and their functional equivalents should not be limited byparticular embodiments described and illustrated herein, as these aremerely exemplary in nature.

1. A system comprising: a structural element; at least one helicalstrake about the structural element; and at least one ramp to provide atransition from the structural element to the helical strake.
 2. Thesystem of claim 1, wherein the structural element is selected from thegroup consisting of a shell, a collar, an oil flowline, a pipeline, adrilling riser, a production riser, a steel tubular, import and exportrisers, subsea pipelines, tendons for tension leg platforms, legs fortraditional fixed and for compliant platforms, space-frame members forplatforms, cables, umbilicals, mooring elements for deepwater platforms,hull structures for tension leg platforms and for spar type structures,and column structures for tension leg platforms and for spar typestructures.
 3. The system of claim 1, wherein the structural elementcomprises a plurality of sections welded to each other.
 4. The system ofclaim 1, wherein the structural element comprises a plurality ofsections threaded to each other.
 5. The system of claim 1, wherein theat least one helical strake about the structural element comprises atleast three helical strakes about the structural element.
 6. The systemof claim 1, wherein the at least one ramp comprises a plurality of rampsaligned along a longitudinal axis of the structural element, the rampsadapted to interface with a stinger and/or a roller.
 7. The system ofclaim 1, wherein the at least one ramp comprises a first set of rampsand a second set of ramps, the first set and the second set alignedalong a longitudinal axis of the structural element, the first setadapted to interface with a first roller, and the second set adapted tointerface with a second roller azimuthally spaced apart from the firstroller.
 8. The system of claim 1, wherein a first end of the at leastone helical strake is attached to a first collar, and a second end ofthe at least one helical strake is attached to a second collar, thefirst collar and the second collar attached about the structuralelement.
 9. A method of installing a structural element in a body ofwater comprising: attaching at least one helical strake about thestructural element; attaching at least one ramp to the structuralelement and/or the at least one helical strake, the at least one ramp toprovide a transition from the structural element to the helical strake;and moving the structural element, the ramp, and the strake over aroller, so that the at least one ramp provides a transition from thestructural element to the helical strake where the roller interfaceswith the structural element, the ramp, and the strake.
 10. The method ofclaim 9, wherein the structural element is selected from the groupconsisting of a shell, a collar, an oil flowline, a pipeline, a drillingriser, a production riser, a steel tubular, import and export risers,subsea pipelines, tendons for tension leg platforms, legs fortraditional fixed and for compliant platforms, space-frame members forplatforms, cables, umbilicals, mooring elements for deepwater platforms,hull structures for tension leg platforms and for spar type structures,and column structures for tension leg platforms and for spar typestructures.
 11. The method of claim 9, wherein the structural elementcomprises a plurality of sections welded to each other.
 12. The methodof claim 9, wherein the structural element comprises a plurality ofsections threaded to each other.
 13. The method of claim 9, whereinattaching at least one helical strake about the structural elementcomprises attaching at least three helical strakes about the structuralelement.
 14. The method of claim 9, wherein the at least one rampcomprises a plurality of ramps aligned along a longitudinal axis of thestructural element, where the roller interfaces with the structuralelement.
 15. The method of claim 9, wherein the at least one rampcomprises a first set of ramps and a second set of ramps, the first setand the second set aligned along a longitudinal axis of the structuralelement, the first set adapted to interface with a first roller, and thesecond set adapted to interface with a second roller azimuthally spacedapart from the first roller.
 16. The method of claim 9, wherein thefirst roller is azimuthally spaced apart from the second roller by 90 to150 degrees measured as an arc angle of the structural element.